Gas turbine engine combustion chambers have been developed to limit the production of undesirable combustion product components such as carbon monoxide (CO), carbon dioxide (CO2), unburned hydrocarbons (UHC), nitrogen oxides (NO, NO2, N2O3), smoke and soot or particulate matter. Different combustion chamber designs have been developed to minimise the production of these undesirable combustion product components. One of these combustion chamber designs uses staged combustion of the fuel. A staged combustion chamber includes first, or primary, stage burners and second, or secondary, stage burners. At low speed and/or low power conditions fuel is supplied to the primary stage burners only. At high speed and/or high power conditions fuel is supplied to the primary stage burners and to the secondary stage burners to maintain the undesirable combustion products within desired limits.
It is difficult to control the supply of fuel to the primary and secondary stage burners to allow efficient thermal operation of the gas turbine engine while simultaneously minimising the production of the undesirable combustion product components. It is to be noted that operating with the secondary fuel at low combustion temperatures to reduce the emissions of NOx, may result in incomplete combustion or partial combustion, which may lead to the production of excessive amounts of unburned hydrocarbons (UHC) and carbon monoxide (CO) in addition to producing lower power and lower thermal efficiency. On the contrary it is to be noted that operating without the secondary fuel at high combustion temperatures will reduce the emissions of carbon monoxide (CO) and unburned hydrocarbons (UHC) but often results in higher emissions of NOx and smoke.
Currently the best practice is to supply fuel to the secondary stage burners based on either a sensed measurement of the temperature at the inlet to the combustion chamber or a derived measurement of the temperature at the outlet of the combustion chamber. However, both methods are likely to be inaccurate and rely on certain assumptions.
Therefore the present invention seeks to provide a novel apparatus and a novel method for controlling the supply of fuel to a combustion chamber which reduces or overcomes the above mentioned problem.